It is known from U.S. Pat. Nos. 5,773,305 and 5,114,162 to mix a fluid sample such as blood and a diluent, inside a probe tip by first aspirating both liquids into the tip, and then drawing said liquids further up into the tip into a mixing chamber having an enlarged inside diameter compared to the rest of the tip. The mixing can be achieved, for example, by reciprocating the mass of liquids up and down numerous times.
In the examples shown in U.S. Pat. No. 5,773,305, the liquids are retained in the enlarged chamber and simply sloshed back and forth in that chamber to achieve mixing. FIG. 3 thereof makes it clear that simply aspirating the liquids into the enlarged chamber past a step discontinuity created by the enlarged inside diameter, is ineffective in creating a mixture. That is, a single movement past the step discontinuity is shown as not mixing the fluids homogeneously. An air bubble can also be included between the liquids when first aspirated. Cross-over contamination between bodies of liquid being aspirated is preferably prevented by ejecting an inert oil shield around the outside of the tip, FIGS. 7 through 11 thereof.
Such a construction is generally equivalent to transferring two liquids from a pipette into a larger diameter container (the mixing chamber) and attempting mixing by sloshing the liquids vertically within the container. Although mixing can occur in such a fashion for relatively large volumes, it is not as effective for small volumes, e.g., volumes that total 100 to 600 microliters. That is, in a constant diameter channel, inertial mixing is reduced if the volumes are small, as here. It is this phenomenon that requires the movement of the liquids back and forth in the mixing chamber, as much as 20 times, to achieve homogeneous mixing. Such reiterations of the mix step are time-consuming, and beg for an improvement.
Furthermore, it is not the case that cross-contamination is preventable only by using such an oil shield. That is, in some cases, the first-aspirated liquid can be removed from the tip simply be repeated washing with a diluent, or by wiping. In any event, should washing prove to be unsatisfactory, there has been a need for a more reliable method of preventing contamination than by using the oil shield. (The oil shield is not guaranteed to form completely around the tip just because a plurality of dispensing nozzles are disposed about the circumference of the exterior of the tip.) Furthermore, some proteins can destroy the shield effect of the oil.
In the examples of U.S. Pat. No. 5,174,162, all the liquids to be mixed are moved completely into the enlarged mixing chamber, completely out of the chamber, then back into it, and so forth. The sharp transition at surface 15 causes turbulent mixing, 16, FIG. 2 thereof. This is a more efficient mixing method than that of the '305 patent. Nevertheless, there are improvements that are needed in such a mixing system as described in the '162 patent. For example, no optimization is described for the geometry of FIG. 2. Nothing is described regarding any use of air bubbles to separate the liquids as they are aspirated. As noted however in the '305 patent, such an air bubble provides an effective prevention against cross-contamination. Yet, any air bubble must be rapidly eliminated during mixing.
Furthermore, the '162 patent is notably deficient in any teaching to prevent cross-contamination when aspirating liquid 6 immediately after liquid 4, between the two liquids within the bulk container of liquid 6. Although the oil shield of the '305 patent might seem to be applicable to the probe of the '162 patent as well, such a shield has disadvantages as noted above. Alternative protection methods against cross-contamination, besides the oil-shield method, are thus desirable.
Yet another disadvantage of the teachings of the '162 patent is that when the two disparate liquids are moved back and forth across the boundary 15, unmixed “tails” of one or both liquids can be left behind as coatings on either the enlarged chamber or the narrower intake portion. Such residual tails do not get mixed when the main body of liquids is moved across boundary 15, so that the tails are undesirable.
Thus, although substantial development has already occurred in probes designed to mix two liquids entirely with the probe, there remains the need for improvements.